1. Field of the Invention
Embodiments of the invention generally relate to seismic sensing and, in particular, to components and techniques for deploying and interrogating arrays of seismic sensors, such as in ocean bottom seismic sensing (OBS) applications.
2. Description of the Related Art
Marine seismic exploration surveys for the exploration and monitoring of hydrocarbon producing zones and reservoirs utilize seismic cables having sensor arrays, i.e., a plurality of sensor stations interconnected by sections of cable. The cable arrays may include a large number of sensor stations (e.g., several hundreds or thousands) and may be buried in a predetermined pattern on the ocean floor. Optical sensors may be particularly well suited for ocean bottom seismic (OBS) applications, due to their robust nature, lack of sensitive electronics, and potential for light weight sensors and cable assemblies that are relatively inexpensive to install. An optical sensor station may include optical hydrophones, accelerometers along multiple axes, and/or geophones.
The individual sensors in a station, such as accelerometers oriented in orthogonal X, Y, and Z axes, may be interferometers. In such systems, a light source generates interrogating light pulse pairs (spaced apart in accordance with a length of fiber between reflectors in each interferometric sensor), resulting in interfering signals reflected back to the surface. These interfering signals may be analyzed by surface electronics, and recorded and interpreted into seismic data.
As the total number of sensors in the arrays increases in high channel count (HCC) applications, it becomes a challenge to interrogate each sensor using a manageable number of optical fibers run to and from surface instrumentation. While multiplexing techniques, such as wavelength division multiplexing (WDM) and time division multiplexing (TDM) are well known, there are typically limits to each. On the one hand, there is a practical limit as to how many sensors may be interrogated by a single fiber, due to a limited number of wavelengths and limitations on total transmitted power per fiber set by optical nonlinear interactions. On the other hand, TDM of multiple interferometric sensors using reflectors of a common wavelength are subject to unwanted reflections between sensor elements (causing cross-talk).
In some cases, in order to generate sufficient optical power to interrogate a high number of sensors in an OBS array, relatively expensive components, such as remotely pumped sources and optical amplifiers may be used. Unfortunately, such remotely deployed components are relatively expensive and typically require special pressure sealed housings to be operated at the high pressures seen at the ocean bottom. Replacing failing components remotely located subsea is an expensive and time-consuming process.
Packaging and deployment of OBS sensor arrays also create challenges in order to achieve efficient coupling of the seismic signals to the respective sensors. Station packaging should ensure sufficient protection of the sensors during installation and operation, and should also withstand hydrostatic pressures typical at the ocean bottom (e.g., 50-200 bar). The packaging and station design should ensure high reliability over a relatively long expected lifetime and efficient assembly procedures in order to reduce overall manufacturing costs.
Cutting and splicing data transmission cables/fibers within the cable array at each of the sensor stations increases time and cost while decreasing reliability. Design of the sensor station and/or cable array can affect how many splices are required at each station. Accordingly, any designs or techniques that reduce the number of splices of the data transmission cables/fibers at each station decreases assembly time and cost while increasing reliability of the cable array.
Therefore, there exists a need for an inexpensive and improved OBS sensor system with a large number of sensors, as well as corresponding cable and sensor station components, manufacturing and deployment techniques.